This invention relates generally to systems for transferring fluids, e.g., cryogenic fluids, from a vessel to another location or an end user and more particularly to apparatus that can be used in a low pressure cryogenic storage system for pumping a liquid, compressing a gas or pumping/compressing a combination of liquid and gas at high pressure, with the speed of operation of the apparatus being variable depending on the type of fluid being transferred.
Cryogenic fluids, such as liquified hydrogen, oxygen, nitrogen, argon or liquified air, and liquified hydrocarbons, such as liquified methane, butane, propane or natural gas, are typically stored and transported in pressurized containers. The containers are typically well-insulated and refrigerated to very low temperatures. While many types of pumps/compressors have been designed for transferring fluids between containers or from one container to a point of use, mechanical pumps of the reciprocating piston type have been preferred for many applications. Such pumps/compressors usually make use of a motor having a rotary output shaft for driving or reciprocating the piston.
As will be appreciated by those skilled in the art the term “pump” is generally used in the context of liquid handling, wherein the operation of a “pump” increases the pressure on the liquid. The term “compressor” on the other hand is generally used in the context of gas handling, wherein the operation of a “compressor” increases the pressure on the gas. A problem with utilizing a compressor as a pump, or visa versa, stems from the restriction of the pump valves to fluid flow. For a given reciprocating speed, liquid flow through the pump valves will have a much higher pressure drop when compared to gas flow. To make the valves work properly for gas flow, the valves will be too small and restrictive when used for liquid flow. The solution is to control the phase of the fluid going into the pump along with pump rotational speed. If only gas is admitted, the compressor can run at a higher speed compared to if liquid is admitted to the pump.
A second problem arises when considering the power required for pumping a liquid as compared to compressing a gas. For any given rotational speed, the pump will need much more power than a compressor. A variable speed motor does not resolve this issue, since the power a variable speed motor puts out is directly proportional to rotational speed. Thus, at low rotational speed, where liquid must be pumped, the power required is highest, but the motor's power output is lowest.
A third problem regards the control of piston ring blowby gas. For a cryogenic pump being used for liquid, blowby liquid will flash and at least partially convert to a gas. If this blowby fluid is routed back to the pump suction, the gas displaces some liquid, thus reducing the flow rate of the pump which may be seen as a detriment to system performance. If the reciprocating machine is being used to compress a gas, there may be a negligible effect on system performance if this blowby gas is re-ingested. Thus routing of blowby gas in a system which uses a single machine to pump liquid and also to compress gas is a concern, and a method of controlling blowby is beneficial.
In order to pump a liquid or mostly liquid fluid, the rotational or reciprocating speed of the motor driving a reciprocating piston pump should be relatively low, e.g., 600 rpm or less, primarily due to the pump's valves and its power requirements. To compress a gas, or mostly gas fluid, a reciprocating piston compressor will benefit from a higher rotational or reciprocating speed, e.g., 1200 rpm.
Because boiling and vaporizing of a cryogenic liquid in a storage tank results in a gas which must be dealt with, various techniques have been proposed for pumping the liquid and compressing the gas. For example, one technique of the prior art focuses on separating the liquid from the gas, and pumping/compressing each component separately using different devices. In U.S. Pat. Nos. 6,171,074 (Charron), U.S. Pat. No. 6,273,674 (Charron), and U.S. Pat. No. 6,296,690 (Charron) there are disclosed various methods and apparatus for separating a mixture of liquid and gas in oil field applications. In the case of a cryogenic tank, heat from the environment causes some of the liquid to boil off, thereby increasing the tank pressure unless the gas is vented. In such a tank, the mixture of gas and liquid may be controlled by piping vapor (gas) from the top of the tank and liquid from the bottom of the tank. In such an arrangement, the ability to compress excess gas in the tank in order to prevent over pressurization and loss of product by venting is highly desirable. To that end, U.S. Pat. No. 4,447,195 (Schuck), U.S. Pat. No. 5,243,821 (Schuck et al.) and U.S. Pat. No. 6,640,556 (Ursan et al.) describe methods of mixing vapor with liquid.
While the above patents appear generally suitable for their intended purposes, they never the less leave much to be desired from the standpoint of their ability to deliver a cryogenic product as well as conserving and reducing venting and associated wastage of the product. A single reciprocating piston device capable of handling both liquid and gas of a cryogenic product is thus desirable. Unfortunately, prior to the subject invention there hasn't been a single pump/compressor apparatus that can accomplish the desired pumping/compressing of a liquid/gas on a viable, cost effective basis.
As will be seen from the discussion to follow, the subject invention accomplishes those ends by a simple methodology, e.g., changing the reciprocating speed of the pump/compressor to accommodate the particular fluid being pumped. Changing the operating speed of a pump or fan has been described in U.S. Pat. No. 5,947,854 (Kopko), but not in the context of pumping/compressing two different phase fluids.